1. Field of the Invention
The present invention relates to a plane mechanical keyboard designed to be integrated into microcomputer type pocket computers or electronic devices or into portable telephones for example.
2. Description of the Prior Art
Mechanical keyboards are widely available in the market. They include membrane-type keyboards, flexible contact keyboards and also touch pad keyboards.
However, presently existing mechanical keyboards have keys that are far too small to enable the high-speed and efficient typing of a text. For, since the keys have a size that is generally smaller than the contact surface of a user's finger, it becomes impossible to press a key without rubbing against at least one of its angular edges. Consequently, the high-speed and prolonged typing of a text very soon becomes irksome and ergonomically unsound.
Furthermore, the small sizes of the keys and the small interstices between two contiguous keys mean that the typing must be done with very great precision in order to prevent the many typing errors that are likely to occur.
FIGS. 1A to 1C show three types of prior art keyboards. These three keyboards are made to the same dimensions, and the spacing E between the centers of two neighboring keys is constant from one keyboard to another. Only the width of the keys, respectively referenced l.sub.A, l.sub.B and l.sub.C, varies from one keyboard to another. Now, it is this dimension of the keys that plays a very great role in the value of the margin of error available to the user around the center of a key of a keyboard. The margin of error is geometrically defined as the size of a horizontal or vertical segment on which the center of the finger must be placed in order that the striking of the required key may be valid. This margin is generally inversely proportional to the striking precision.
In general, the value of the spacing E between the centers of two neighboring keys for small-sized keyboards designed for pocket devices ranges from 1 to 1.5 cm.
Furthermore, it is assumed by approximation that the contact surface of an adult finger on a keyboard, when a key is struck, forms a circle whose diameter, designated by the reference d, is estimated at about 0.8 cm. In order that a user may be sure of being able to depress one of the keys of a keyboard by randomly pressing on this keyboard, it is furthermore necessary that the width of the keys of this keyboard should range from E to E-d, namely about 0.2 and 0.7 cm.
FIGS. 1A to 1C show the development of the margin of error, referenced m.sub.A, m.sub.B and m.sub.C, as a function of the width of the keys. It can be seen that, when the width l.sub.C of the keys is equal to E, the margin of error m.sub.C is the minimum, namely a user must take great care to avoid striking two keys simultaneously.
However, when the width l.sub.A of the keys is the minimum and equal to E-d, then the margin of error is the maximum. In this case indeed, no great precision is required to strike the keys since they are sufficiently spaced out to prevent the possibility that two keys may be struck simultaneously. FIG. 1B illustrates an intermediate example with a value of l.sub.B ranging from E to E-d.
However, the optimum approach shown in FIG. 1A is not ergonomically sound. For, as described earlier, since the striking surface l.sup.2 is smaller than the contact surface (.PI.d.sup.2 /4) of a user's finger, it is impossible to strike a key without rubbing against at least one of its angular edges. Consequently, this type of key cannot be used for the fast and prolonged typing of a text in an efficient manner.